1. Field of the Invention
The present invention relates to a multipath signal searcher operating in a wireless communication system for obtaining a mobile communication signal and, more particularly, to a method of performing a mobile communication signal search over a search window of time offsets.
2. Descriptions of the Related Art
In wireless communication systems, a mobile communication signal transmitted between a mobile station (MS), such as a mobile telephone or a cellular telephone, to a base transceiver station (BTS) may be reflected off numerous scattering means, such as buildings, before reaching the BTS. As a result of these reflections, the mobile communication signal from the MS may propagate over multiple paths before reaching the BTS, for example. These reflected signals, typically referred to as multipath replicas, represent replicas of the original mobile communication signal and arrive at the BTS with variations in signal power levels, and at varying times, due to varying signal propagation lengths. Upon receipt by the BTS, the originally transmitted communication signal and the multipath replicas are filtered, despread, recombined and decoded to yield the desired voice or data signal originally sent by the MS. The same communication process described above between the MS and the BTS (reverse link) also occurs between the BTS and the MS (forward link).
One type of a wireless communication system is a direct sequence code division multiple access (DS-CDMA) cellular communication system such as set forth in the Telecommunication Industry Association Interim Standards 95 and 2000, hereinafter referred to as IS-95 and IS-2000, respectively. The mobile coded communication signals transmitted to and from the mobile station within the IS-95 or IS-2000 standards comprise spread spectrum signals which are transmitted in a common 1.25 MHz bandwidth channel, where each 1.25 MHz bandwidth channel is commonly referred to as a narrowband DS-CDMA channel. An exemplary narrowband code division multiple access (CDMA) communication system that may be used with the IS-95 standard is described in U.S. Pat. No. 6,108,324 to Brown et al. entitled APPARATUS AND METHOD FOR PERFORMING A SIGNAL SEARCH IN A COHERENT WIRELESS COMMUNICATION SYSTEM, which is incorporated herein by reference. In addition, there are numerous international standards proposed which suggest a coherent wideband CDMA transmission signal configuration that can be used to transmit other data in addition to the voice and low rate data of narrowband CDMA systems. An exemplary wideband CDMA communication system that may be used with the IS-2000 standard is described in U.S. Pat. No. 6,125,137 to Wang et al. entitled APPARATUS AND METHOD FOR PERFORMING A SIGNAL SEARCH IN A WIRELESS COMMUNICATION SYSTEM, which is incorporated herein by reference.
To search for the mobile communication signal, a multipath signal searcher is employed within the BTS or the MS, along with a RAKE receiver in communication with the multipath signal searcher. The mobile communication signal is searched over a search window having a plurality of time offsets. The multipath signal searcher includes numerous search paths, each separately searching for the mobile communication signal at a particular time offset within the search window. The time offsets correspond to pn-offsets in CDMA systems, and the resolution of these time offsets is typically one half of the PN chip duration (Tc). In typical systems, the search paths within a multipath signal searcher function in either serial or parallel modes.
In serial operation, the multipath signal searcher searches through the first half of the search window for the mobile communication signal by having all the search paths within the multipath signal searcher search only those time offsets over the first half of the search window. The first half is searched in one search time slot and the remaining portion of the search window, or second half of the search window, is searched in the next search time slot. For example, a search window may span 40:s corresponding to 50 time offsets for a PN chip rate of 1.2288xc3x97106 chips/sec where there are two time offsets per PN chip. The multipath signal searcher in serial operation would search the first 25 time offsets, i.e., offsets 0-24, in one time slot and the second 25 time offsets in the next time slot, i.e., offsets 25-49. As a result, the finger manager which oversees the filtering, despreading, recombining and decoding of the multipath signal uses the search results from the multipath signal searcher to update the finger status at a rate of once every two time slots.
FIG. 1 is a plot of an integration period used in a conventional mobile communication signal serial search. The time it takes for the multipath signal searcher to search is shown. The first half of the 50 time offsets are searched in the first search time slot, i.e., integration period 10 of four power control groups (PCGs) in length, in the example provided. In the IS-95 reverse link standard, the integration period 10 can represent the entire non-coherent accumulation of a search metric over the integration period 10. In the IS-2000 standard, the integration period 10 can represent, for example, that a search metric may be coherently accumulated within four different PCGs 12, 14 ,16 ,18 and non-coherently accumulated over the entire integration period 10. It takes a second integration period 20 to search the remaining time offsets, where the search path performs identically as over the first integration period 10, i.e., in the IS-2000 standard, the search metric may be coherently accumulated over four different PCGs 22, 24, 26, 28 and then non-coherently accumulated over the integration period 20. Therefore, in conventional serial operation, when searching the signal received from one antenna it takes the multipath signal searcher a total of eight PCGs, or two integration periods (10 and 20) to search the entire search window and thereafter be ready to update the fingers.
With parallel operation, the searcher is able to search the entire window in one search time slot; the fingers can then be updated every time slot. As shown in FIG. 2, the complete 50 time offset search window is searched in one integration period 30 and the finger manager, and therefore the finger demodulators in communication therewith, may be updated every four PCGs 32, 34, 36, 38. Since a multipath replica signal undetected by the finger demodulator may not only be ineffectual in improving demodulation quality, but potentially acts as interference to the receiver, a fast searcher will help improve receiver performance in a dynamic communication environment. It is therefore desirable to improve the searcher speed and thus the finger update rate to improve wireless communication receiver performance.